Displays are ubiquitous and are a core component of every wearable device, smart phone, tablet, laptop, desktop, TV or display system. Common display technologies today range from Liquid Crystal Displays (LCDs) to more recent Organic Light Emitting Diode (OLED) displays.
Referring now to FIG. 1, there are shown an active drive matrix for a display. The matrix comprises N rows of cells divided into M columns. Each cell includes a light emitting device corresponding to either: a pixel for a monochrome display; or one of a red, green or blue sub-pixel within a color display. For color displays either: differently colored sub-pixels can be interleaved along rows of the matrix; or respective rows of the matrix can comprise only sub-pixels of a given colour.
A plurality of peripheral driving blocks comprise:
Scan driver—which produces pulsed signals S1 . . . Sn enabling respective rows of the matrix to be programmed for a subsequent frame or sub-frame; and
Data driver—which delivers data outputs D1 . . . Dm to program individual cells of a row enabled by the scan driver—these signals are updated for each frame or sub-frame from scan line to scan line.
In some matrices, a constant supply voltage (Vdd) is provided to each cell of the matrix to drive the light emitting device during a frame according to the cell programming. Typically, for a constant supply voltage (Vdd) implementation, the data driver provides analog outputs which determine the brightness of a cell for a subsequent frame.
In the matrix of FIG. 1, a PWM (Pulse Width Modulation) Driver produces PWM pulses used to bias programmed cells enabling the cells to emit light or not during a sub-frame according to their programming. (Note that the term “PWM” is used in the present description to relate to pulsed signals for activating cells within a row—such pulses may be employed as part of a conventional PWM addressing scheme, such as described in WO2010/014991 or a color sequential scheme, such as described in WO2014/012247.) For PWM, the data driver typically provides digital outputs with the PWM driver providing variable width pulses which in combination with the cell programming for a sub-frame determines the brightness of a cell for a frame.
UK Patent Application No. 1604699.7 (Ref: 135-1702-01GB) filed 21 Mar. 2016 discloses a hybrid scheme where the data driver provides combinations of analog or digital outputs limiting the switching frequency required of the PWM driver.
In FIG. 1, two synchronization blocks are employed: one located between the scan driver and data driver in order to ensure that the required data signals are delivered after a scan pulse is applied to a row; and a second between the data and PWM drivers to ensure that PWM pulses are applied when data loading is completed.
Each row within the matrix is addressed with a respective scan line S1 . . . Sn which goes high or is asserted when a respective row of the display is to be addressed (or programmed) by the data driver for the subsequent frame or sub-frame. For PWM, during a given frame, for each row, the PWM driver provides a sequence of driving pulses using respective PWM signals P1 . . . Pn. Each signal P can be a time shifted version of the adjacent PWM signal synchronized with the scan line signals S1 . . . Sn and data driver signals D1 . . . Dm.
Active matrix circuitry, for example, as described in WO2010/119113, uses thin film transistor technology (TFT), where cells comprise transistors based on amorphous, oxide or polycrystalline silicon technology manufactured on a glass substrate ranging in size from 30 cm×40 cm to the latest generation (known as GEN10) of 2.88 m×3.15 m. The TFTs are used either as voltage switches or current sources to control the operation of light emitting devices within each cell.
In most portable, typically battery powered, devices, the display uses the majority of the available power. The most common user complaint for portable devices is insufficient display brightness. To extend battery life and improve brightness levels it is necessary to develop new display technologies that reduce power consumption and produce higher luminance emission from the light source.
WO2013/121051 discloses an improved light emitting device for a display, referred to as an integrated or inorganic LED (iLED) which comprises a substrate with a semiconductor material comprising a light generating layer positioned on the substrate. The semiconductor material and/or the substrate are configured to control light internally to output quasi-collimated light from a light emitting surface of the iLED. The iLED comprises an optical component positioned at the light emitting surface and configured to receive quasi-collimated light exiting the light emitting surface and to alter one or more optical properties of at least some of the quasi-collimated light.
Whereas OLED cells operate by passing current through organic or polymer materials sandwiched between two glass planes to produce light; iLED displays replace the OLED material with discrete LED die (which is made of inorganic materials) placed at each cell of the display.
Nonetheless, both OLED and iLED cells are current driven. This means that their emitted brightness is controlled by the current that flows through them, so the stability of the biasing current across the display will determine the uniformity of light emitted from the display.
Referring now to FIG. 2, a typical 2-TFT-1-capacitance (2T1C) pixel design for a constant supply (i.e. non-PWM) active matrix display is shown. In this case, T1 acts as a switch and T2 is the driving TFT that produces the bias current for the light emitting device. During a frame programming period, the scan signal goes high “1” and T1 is turned ON and the storage capacitance Cst is charged up to Vdata—the voltage provided by the data driver. T2 operates within its saturation region and the voltage at node A (which is equal to Vdata) is its gate voltage. Therefore, its drain current and bias current will be:
                              I          drain                =                              I            bias                    =                                    W              L                        ⁢            μ            ⁢                                                  ⁢                                                            C                  ox                                ⁡                                  (                                                            V                                              gs                        ,                                                  T                          ⁢                                                                                                          ⁢                          2                                                                                      -                                          V                      th                                                        )                                            2                                                          (        1        )            
where W and L are the gate width and length, respectively, μ is the carriers mobility, Cox is the gate-oxide capacitance, Vgs is the gate-to-source voltage and Vth is the threshold voltage of the TFT device. Another way of expressing the above is:
                                          I            bias                    =                                    k              ⁡                              (                                                      V                    data                                    -                                      V                    th                                                  )                                      2                          ,                              where            ⁢                                                  ⁢            k                    =                                    W              L                        ⁢            μ            ⁢                                                  ⁢                          C              ox                                                          (        2        )            
As indicated above, TFT devices can be either amorphous silicon (a-Si), Indium-Gallium-Zinc-Oxide (IGZO), Low-Temperature polycrystalline silicon (LTPS) or organic (OTFTs). Depending on the fabrication process, threshold voltage variations occur either during fabrication (LTPS) or during operation, under positive bias stress (A-Si, IGZO, OTFT). The threshold voltage variation can be regarded as a completely random process and can exist even for TFT devices fabricated on the same substrate.
Thus, for a display where cells are programmed with the same Vdata during the frame refresh so that two cells might emit the same grey scale (same light brightness), their driving TFTs can have different threshold voltages. The produced bias current will be different since Vth1≠Vth2→Idrain1≠Idrain2, resulting in different emitting brightness. This non-uniformity of brightness caused by threshold voltage variations of the TFT is called mura effect.